1. Field of the Invention
The present application relates to a multi-service communications system in which a plurality of different services are transmitted to subscribers over one or more different frequency channels. More particularly, the present invention relates to the transmission of a Logical Channel Table (LCT) which provides an abstraction between network parameters, such as transport stream ID and program number, and application layer elements, such as program guides or numeric channel selections, and to the transmission of a Composite Channel Table (CCT) which supports a time-sequenced concatenation of services from possibly different multiplexes so that the concatenation of services appears as a single service requiring no intervention on the part of the subscriber.
2. Description of the Prior Art
In accordance with the MPEG-2 Systems Standard (ISO/IEC 13818-1), one or more programs are combined into a single transport stream for transmission. Data from each elementary stream are multiplexed together with information that allows synchronized presentation of the elementary streams within a program. Generally, a transport stream consists of one or more programs, and the audio and video elementary streams consist of access units. As known to those familiar with the MPEG-2 Systems Standard (ISO/IEC 13818-1), a program is a collection of elementary streams with a common timebase. In other words, a program consists of all the elementary streams which refer to a common Program Clock Reference (PCR) clock. The elementary stream data is carried in Packetized Elementary Stream (PES) packets, where a PES packet consists of a PES packet header followed by packet data. The PES packets are inserted into transport stream packets for transmission. The PES packet header may contain decoding and presentation time stamps (DTS and PTS) as well as other optional fields. Transport stream packets, on the other hand, begin with a 4 byte prefix containing the 13 bit packet ID (PID). The PID identifies, via four Program Specific Information (PSI) tables, the contents of the data contained in the transport stream packet payload.
In the context of the MPEG-2 Systems Standard (ISO/IEC 13818-1) transport stream, acquiring programs requires information from each of the four Program Specific Information (PSI) tables defined by the MPEG-2 Systems Standard (ISO/IEC 13818-1) specification. These tables are the Network Information Table (NIT), the Program Association Table (PAT), the Program Map Table (PMT) and the Conditional Access Table (CAT), and are typically segmented into sections and inserted in transport stream packets. Upon receipt at the decoder, these tables are searched for the desired parameters and need not necessarily be stored locally since they are frequently retransmitted.
The NIT specifies the mapping between transport stream IDs and network physical parameters such as tuning frequencies, transponder numbers, and the like. If an NIT is present, it must be encapsulated in an MPEG-2 private section. The PAT specifies the packet identifiers (PIDs) for the packets which carry Program Map Tables (PMTs) for the components of one or more programs on a transport stream. In other words, the PAT associates a program number with the transport packets that carry the PMT for that program. The PAT is always sent in packets with PID=0. The PMT specifies the PIDs and therefore which elementary streams and descriptors are associated to form each program. The PMT also indicates the PID of the transport stream packets which carry the Program Clock Reference (PCR) for each program, where the PCR is a "snapshot" of the original 27 MHz system time clock of the program. The PMT further indicates stream types, elementary PIDs, program numbers of "virtual channels," conditional access entitlement control messages (ECMs), and the like of packets that make up a particular program. The CAT is used when scrambling is employed and specifies the PIDs of conditional access packets used by decoders to gain entitlements for programs on transport streams. The CAT associates one or more Entitlement Management Message (EMM) and Entitlement Control Message (ECM) streams and other conditional access data with a unique PID and uses a CA descriptor to specify CA linkages and private data. The CAT is always found in PID=1. Several other related structures have been defined by the Digital Video Broadcasters (DVB) group to provide descriptions and access methods for services. These structures and related information are known collectively as the DVB Service Information (SI) and are currently specified as ETSI prETS 300 468 (Draft --November 1994). Further information regarding the above-referenced tables may be found by referring to the MPEG-2 Systems Standard (ISO/IEC 13818-1), the contents of which are hereby incorporated by reference.
In addition to these existing MPEG-2 Systems Standards (ISO/IEC 13818-1) and DVB structures, there are at least two other sources of information that might be used in the tuning process, namely, selections from Electronic Programming Guides (EPG) and direct "channel" selection by the subscriber. As known by those skilled in the art, the EPG is an application that allows subscribers to discover and select their desired programs. The EPG provides the mapping from a human-readable program name, icon, and the like to a specific program as defined, for example, by the MPEG-2 Systems Standard (ISO/IEC 13818-1). The EPG typically needs the subscriber to select a service, which, in turn, identifies the program number and transport stream ID of the selected program. The format and content of typical EPGs are known to those skilled in the art. For example, a description of an EPG assigned to the same assignee as the present invention is given by Hamilton et al. in U.S. patent application Ser. No. 08/072,911, filed Jun. 7, 1993, and entitled "Electronic Program Guide and Text Channel Data Controller." However, in simpler applications, program selections may be accomplished through direct selection of the "channel" number by the subscriber. This could be achieved, for example, via a numeric keypad remote control where the subscriber selects a number which represents a "channel" (or service) and enters that number directly into the set top unit (STU).
To support "channel hopping" by the subscriber, a list of "contiguous" channels must be available. That is, there must exist a way to associate individual channels together as an ordered list so that "up/down", ".+-." or "increment/decrement" keys will cause the set top unit to tune to the "next" channel in the program listing.
In addition to the access modes described above, it is desirable to support services which are "composites" of other services. That is, a subscriber may subscribe to a service which is composed of, for example, all football games on the network, even though those games are sourced from different individual services such as off-air channels, satellite-distributed sports networks, or digital storage media (for "classic" games, for example). This composite service should be "transparent" so that no intervention for re-tuning is required of the subscriber in order to receive all such programming. A system of this type is described in U.S. Pat. Nos. 5,359,601 and 5,418,782, both also assigned to the same assignee as the present invention.
However, in order to adapt the MPEG-2 Systems Standard (ISO/IEC 13818-1) to accommodate the subscriber selections from an EPG and program selections including composite services, additional structures to those mentioned above for use in MPEG-2 systems are required. This is so because the service provider desires the flexibility of moving programs around to different transport streams and different frequencies without having to update the EPG service provider of each and every change. The present invention has been developed to provide such structures so that the desired functionality can be provided in a manner that is "transparent" to the subscriber as well as to the EPG service provider.